feat: ADR-0035 4a.2 — per-column CHECK/DEFAULT + composite UNIQUE

Advanced-mode SQL CREATE TABLE gains the constraints that need no new
internal table (the 4a.2 slice):

- Grammar (sql_create_table.rs): column-level DEFAULT/CHECK and
  table-level UNIQUE(cols). DEFAULT is a literal or a *parenthesised*
  expression (standard SQL) — a bare sql_expr greedily eats a following
  NOT (NOT IN/LIKE/BETWEEN), breaking `DEFAULT 0 NOT NULL`; the parens
  bound it. CHECK is paren-bounded already.
- Builder (ddl.rs): captures CHECK/DEFAULT raw SQL text by byte span
  (sql_expr builds no AST) via capture_parenthesised_span /
  capture_expr_span; routes single-column table UNIQUE into the
  column's flag and composite UNIQUE into unique_constraints.
- Command/worker: ColumnSpec gains check_sql/default_sql (raw, preferred
  over the typed Expr/Value); Command::SqlCreateTable + Request +
  do_create_table gain unique_constraints; do_create_table emits raw
  CHECK/DEFAULT and composite UNIQUE clauses.
- Round-trip (part D): ReadSchema/TableSchema gain unique_constraints;
  read_schema detects composite UNIQUE via PRAGMA index_list origin 'u'
  (single-column still folds to the column flag); schema_to_ddl emits
  them; YAML RawTable/write_table round-trips (optional-on-read).
  CHECK round-trips via __rdbms_playground_columns.check_expr, DEFAULT
  via PRAGMA table_info — no new metadata table.

Table-level/multi-column CHECK remains 4a.3 (rejected "not yet
supported"); FK is 4b.

Tests: +7 builder (raw-text capture incl. the DEFAULT 0 NOT NULL
boundary the fix was found by; single/composite UNIQUE routing) and +4
Tier-3 (CHECK enforced, DEFAULT applied, composite UNIQUE enforced, and
all three survive a rebuild — the part-D round-trip). 1752 pass / 0 fail
/ 1 ignored; clippy clean. Plan + requirements.md updated.

src/persistence/mod.rs

@@ -140,6 +140,12 @@ pub struct TableSchema {
     pub name: String,
     pub primary_key: Vec<String>,
     pub columns: Vec<ColumnSchema>,
+    /// Composite (multi-column) `UNIQUE` constraints (ADR-0035
+    /// §4a.2). Single-column UNIQUE is carried on the column's
+    /// `ColumnSchema::unique` flag instead. Empty for project files
+    /// written before composite UNIQUE existed — the YAML field is
+    /// optional on read.
+    pub unique_constraints: Vec<Vec<String>>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]